Interpretive Summary: Phosphorus is a major plant nutrient and essential for photosynthetic processes in leaves. It is an important limiting factor for cotton growth and development in many soil types worldwide. The atmospheric carbon dioxide (CO2) is projected to double by the end of 21st century. Generally, high atmospheric CO2 increases plant photosynthesis and growth. However, the availability of plant nutrient, such as phosphorus in soil, controls the overall beneficial effect of high CO2. The results showed that under phosphorus deficiency, high CO2 reduced the photosynthetic limitations caused by diffusion but failed to minimize the biochemical limitations. This work illustrated that the projected high CO2 may not increase the photosynthetic capacity of cotton plant under phosphorus deficiency. However, the small beneficial effect of high CO2 on plant biomass under phosphorus deficiency was mainly caused by increased plant height, leaf number and larger leaf area leading associated increase in total canopy photosynthesis.

Technical Abstract:
Nutrients such as phosphorus availability may exert a major control over plant response to rising atmospheric carbon dioxide concentration (CO2), which is projected to double by the end of 21st century. Elevated CO2 may overcome the diffusional limitation to photosynthesis posed by stomata and mesophyll and alter the photo-biochemical limitations resulting from phosphorus deficient conditions. To evaluate these ideas, cotton (Gossypium hirsutum L., cv. deltapine 555) was grown in controlled environment growth chambers with three levels of phosphate (Pi) supply (0.2, 0.05 and 0.01 mM) and two levels of CO2 concentration (ambient 400 and elevated 800 µmol mol-1) under optimum temperature and irrigation. Phosphate deficiency drastically inhibited photosynthetic characteristics and decreased cotton growth for both CO2 treatments. Under Pi stress, an apparent limitation to the photosynthetic potential was evident by CO2 diffusion through stomata and mesophyll, impairment of photosystem functioning and inhibition of biochemical process including the carboxylation efficiency of rubulose-1,5-bisphosphate corboxylase/oxyganase and rate of rubulose-1,5-bisphosphate regeneration. The diffusional limitation posed by mesophyll was up to 58% greater than stomatal conductance (gs) under Pi stress. As expected, elevated CO2 reduced these diffusional limitations to photosynthesis across Pi levels however, it failed to minimize the photo-biochemical limitations to photosynthesis in phosphorus deficient plants. Acclimation/down regulation of photosynthetic capacity was evident under elevated CO2 across Pi treatments. Despite a decrease in phosphorus, nitrogen and chlorophyll concentration in leaf tissue and reduced stomatal conductance at elevated CO2, the rate of photosynthesis per unit leaf area when measured at the growth CO2 concentration tended to be higher for all except the lowest Pi treatment. Nevertheless, plant biomass increased at elevated CO2 across Pi nutrition with taller plants, increased leaf number and larger leaf area.